Hi Jen,
In order to do that for existing work items you'll have to iterate through existing work items using a scheduled workflow rule. Create a new SWR on the work item level and set it to run once year (or just disable it after it worked). Put in the evaluation criteria $C_CUSTOMFIELDNAME=NULL . In the action update the field based on the same logic that you have for the default value.
Please let me know you you were successful.
Thanks!
Tamir